Fiddler Classic crashes on startup complaining about the "Configuration System":

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    Configuration system failed to initialize
    Source: System.Configuration
    at System.Configuration.ConfigurationManager.PrepareConfigSystem()
    at System.Configuration.ConfigurationManager.GetSection(String sectionName)

    System.Configuration.ConfigurationErrorsException: Unrecognized configuration section system.serviceModel. (c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Config\machine.config line 134)
    or

    System.Configuration.ConfigurationErrorsException: Unrecognized configuration section runtime. (C:\Program Files (x86)\Fiddler2\Fiddler.exe.Config line 2)

This error message indicates that one of the .NET Framework's configuration files is corrupt. The most common fix for this is to visit WindowsUpdate and install all available .NET Framework updates. If that doesn't work, try re-installing the .NET Framework. If that doesn't work, try editing the file specified in the error message to correct whatever the error message is complaining about.